Dora represents the idealized figure of female beauty, while Agnes is a practical beauty.
David’s first wife, Dora, is angelic. She is young, small, cheerful, helpless, and pretty. She is also silly and weak. She can’t keep house, and she certainly can’t survive childbirth. She is a doll more than a woman. David’s first description of Dora says it all.
She was more than human to me. She was a Fairy, a Sylph, I don't know what she was - anything that no one ever saw, and everything that everybody ever wanted. I was swallowed up in an abyss of love in an instant. (Ch. 26)
This is a physical attraction, and David idolizes Dora. She can do no wrong in his eyes, even though she is an irresponsible little thing sometimes. She does not last very long because she is too perfect. Youth and beauty were her strengths, and she was spared the pain of losing them by dying young.
Dora’s death gave David a chance to choose a wife more wisely the second time. This time he chose someone who was more practical. Agnes was simple and kind, and his intellectual equal. His attraction to her was her personality, not her looks. This is something a wife can sustain, unlike beauty. Still, it takes David quite a while to realize that Agnes is “the one” and marry her.
I see her, with her modest, orderly, placid manner, and I hear her beautiful calm voice, as I write these words. The influence for all good, which she came to exercise over me at a later time, begins already to descend upon my breast. (Ch. 16)
David gets to right a wrong in marrying Agnes. He marries the right person the second time around.
There are many autobiographical elements to David Copperfield’s story. Dickens seems to have felt that he married the wrong person. He married his Dora, but he wished he had married an Agnes. Dora was young, beautiful, and fragile. None of those traits can last. Marrying for them is bound to end in heartbreak. Dickens gave David a gift, having silly Dora die young and beautiful. David wasn’t saddled with her when she became old and fat, but remained silly.
Dora Spenlow and Agnew Wickfield were both wives to David. Dora, David’s first wife, is foolish and giddy. David calls her his "child-wife" because she is young and childish. She is more interested in playing with her dog, Jip, than in doing household chores. She is not at all competent at household chores.
Agnes Wickfield, David’s second wife, is clam and gentle. She gives support to David when he needs comforting with kind words or advice. It is on this point where the two differ. Agnes is the one who can offer comfort to her husband, while Dora, who is too childish.
Compare dora to his mother, Clara, both child like brides. Compare Agnes to Emily, both sisters. contrast the neglect Clara and Dora experience from their relations which leave them vulnerable to abusive psychopaths, like Heep and Murdstone. Compare Emily and Agnes to lost passionate sisters of David who leaves them wanting love, which they recognize in David, but do not get.
David loves both women differently based on the women he loved as achild, Clara, his mother, and Emily, his, as he called her, sister. Dora because she is like his mother, Agnes, because she is like Emily.
Contrast Agnes with Dora by their different upbringing, which we don't know about except in how they turned out. Compare them by seeing the similarities with how they succumb to a strong man, whether a devient like Heep or Murdstone, and how they groom men who love them to be kind and tender, like David in both cases. The comparision and contrast should include how different they were groomed to be and their personalities vs. how similar were their choices in abusive men. The conclusion might be how oppresive the culture of the times was that two very different types of women, with oppostite upbringings were encouraged by society to succumb to abuse in order to remain faithful to the archetype of what a woman is supposed to be in that society.
Look for these kinds of comparisons and contrastings and continue on with your research paper. Do not expect others to write it for you.